It is often expedient to remove coal from a seam deposit by means of an auger-type cutting assembly comprising a rotatable shaft having a cutting head at one end thereof and a helical vane (or flight) extending along the shaft for conveying coal cut by the cutting head. Holes cut in a vertical mine wall by this type of drilling tool are spaced apart to leave support webs in the mined coal seam. Consequently, a considerable amount of coal is left between holes drilled in this manner. The amount of coal recoverable by a mining operation of the described type could be increased if drill holes could be enlarged at a predetermined distance from a vertical mine facing, thus forming a strong support wall extending for a short distance from the latter and decreasing the web thickness between holes behind this support wall.